The latter approach is the one I'd use, but there may be some pro plumbers that would have other ideas. Only thing about cutting cast iron pipe is that it's nasty stuff to work with. If you have access to a metal grinder, that will do it. Be aware that there is alot of dust put into the air and be very careful with the grinder. The wheel can get hung up in the kerf and jerk out of your hands if you don't have a good grasp. For one cut, this is the way I'd go, but then I own a grinder. Recip saws are supposed to work, but I never had any success with mine. There are also snap cutters that can be rented, but they require a certain amount of space to operate. If you go this route, then use a Fernco coupler to connect PVC pipe and a closet bend to the cast.